Applications such as wide area surveillance and threat detection are driving the need for infrared (IR) camera technology with high density focal plane arrays (FPAs) with large formats greater than, for example, 5 k×5 k pixels and with less than, for example, 10 μm×10 μm pixel size. Conventional nontransparent metal contacts in 3D IR detectors partially cover the active area of the IR detector, resulting in a decrease of external quantum efficiency (QE). The impact on external QE is increasingly worse as the pixel size is reduced. Replacing conventional nontransparent metal contacts with wideband transparent conductors (WTC) is critical for achieving small pixels without compromising detector performance. Wideband transparent conductors in focal plane arrays for visible to long wavelength IR (wavelengths ranging from ˜380 nm to 18 μm) detectors can improve detector performance.
In the prior art, an indium bump bonding process has been used for hybridizing and interconnecting sensor FPAs with readout integrated circuits (ROICs). However, even the best present hybridization process has insufficient yield to meet the requirements of FPA cameras with large formats. Further, more than 4000 kg of force would be needed to hybridize large format arrays with ROICs using a bump bonding process, which is well beyond the capability of hybridization tools.
To address the indium bump process limitations, an heterogeneous integration process may be used, which can eliminates the need for indium bump hybridization. Such a heterogeneous integration process has been described in U.S. patent application Ser. No. 14/158,962, filed Jan. 20, 2014, which is incorporated herein as though set forth in full. With this heterogeneous integration process, large format focal plane arrays can be fabricated by directly bonding an IR sensor wafer to a silicon ROIC wafer followed by removal of the sensor wafer substrate. Then pixel level interconnects to the ROIC may be fabricated by etching deep vias, passivating the via sidewalls, and coating the sidewalls of the vias with an electrical conductor. However, electrical contacts and interconnects made of conventional nontransparent metal for through substrate via holes result in a degraded detector fill factor and decreased external quantum efficiency (QE), which limits future scaling of high density (HD), large format FPAs.
In the prior art, Indium tin oxide (ITO) has been well established as a transparent conductor in the visible wavelength range. However, ITO has less than 30% transmittance (Tλ) in the infrared 6 12 μm range with a film sheet resistance (Rs) of 36 Ω/sq., as described by D. S. Ghosh, L. Martinez, S. Giurgola, P. Vergani, and V. Prneri, “Widely transparent electrodes based on ultrathin metals”, Optics Letters, 34, 325 (2009).
T. Chen, T. Ma, R. C. Baker, “Infrared transparent and electrically conductive thin film of In2O3”, Appl. Phys. Lett. 43, 901 (1983) measured a Tλ of about 16% 26% in the 6 12 μm wavelength IR range with a Rs ˜36 Ω/sq. using indium oxide (In2O).
D. S. Ghosh, L. Martinez, S. Giurgola, P. Vergani, and V. Prneri, “Widely transparent electrodes based on ultrathin metals”, Optics Letters, 34, 325 (2009) describe that ultra thin Ni metal films may have a Tλ of about 80% in the 4 25 μm wavelength range with an Rs ˜110 Ω/sq.; however that is rather high for an ohmic contact.
Z. Wu, Z. Chen, X. Du, et al., “Transparent, conductive carbon nanotube films”, Science, 305, 1273 (2004) describe that carbon nanotube (CNT) films may have a Tλ of about 15% 80% in the 1 12 μm wavelength range with an Rs ˜30 Ω/sq.; however, the wide variation in IR transparency, which is due to the electronic band structure of CNTs, is a major drawback for an IR transparent conductor application. In general, carbon based materials, including CNTs, have not been demonstrated to form ohmic contacts.
What is needed are electrical contacts and interconnects for focal plane arrays that have an improved transmittance (Tλ) in the visible to infrared range with a low Rs. Also needed are electrical contacts and interconnects that do not result in a degraded detector fill factor and decreased external quantum efficiency (QE). The embodiments of the present disclosure answer these and other needs.